Support protocol applications in falcon serve.

[samuel-williams-shopify]

Summary

• Introduce Falcon::Environment::Serve for application configuration discovery and loading.
• Prefer config/serve.rb as a protocol middleware builder configuration, falling back to Rack config.ru.
• Select protocol or Rack loading for explicit --config paths from the .rb or .ru extension.
• Preserve the existing Rack-oriented Falcon::Server.middleware interface while adding protocol-native wrapping.

Testing

• bundle exec sus
• bundle exec bake decode:index:coverage lib
• bundle exec rubocop lib/falcon/server.rb lib/falcon/environment/serve.rb lib/falcon/command/serve.rb test/falcon/environment/serve.rb test/falcon/command/serve.rb